Three years ago the editor of Prospect magazine, David Goodhart, published an article arguing that the increasing diversity, individualism and mobility found in present-day societies may pose a threat to the welfare state. Ethnic diversity produced by immigration adds to this mix. He stirred up a hornets' nest of criticism, even though he was by no means the first to raise the possibility, and indeed he raised only a possibility. The welfare state, he pointed out, is based upon sharing; yet sharing might be in conflict with diversity. People feel stronger obligations to others when they are like themselves.

The very reason, Goodhart argued, why the US has only a minimal welfare state is its diversity, which is much more long-standing than in Europe. A large proportion of the people at the bottom in the US are ethnically different from the majority. 70% of the US population is made up of non-Hispanic whites, but they compose only just over 40% of those living in poverty. Americans think of the poor as members of a different group, whereas in Europe - until recently - they were thought of as part of the same overall community. Multiculturalism and the European welfare state are intrinsically at odds with one-another. Goodhart's thesis was rejected by many critics on essentially ideological grounds - it flouted political correctness by seeming to question multiculturalism.

Another major player has now come into the game, even if his main focus is not the welfare system. Robert Putnam, professor at Harvard, well-known for his work on social capital, has now entered the lists. He has recently published a study based upon a wide-ranging and detailed survey of ethnic diversity carried out in the US. Social capital can be understood as the informal networks of relatives, friends and associates that people depend upon for support in their everyday lives. He found a direct relationship between the homogeneity of neighbourhoods, the level of trust, and the existence of social capital. In those neighbourhoods - such as predominantly white suburbs - where most people are alike, they tend to trust one another more, and also be more involved in community activities, voluntary associations and so forth. In diverse areas, such as inner-city ones, trust and social capital diminish.

Most people wouldn't be particularly surprised by such findings. Like, one could say, attracts like; people feel most comfortable with others who are similar to themselves. However, Putnam also discovered something else that was quite unexpected. In the more diverse communities, there was not simply a retreat into the in-group. In such communities, people have lower levels of trust in those who are different from them, but also in those who are the same. In other words, diversity seems to lower trust and social capital in general. People in the more heterogeneous areas have markedly lower levels of voting, do not trust their local authorities, are less involved in community groups, and are less content with their lives than those in the more uniform ones.

Other factors obviously might explain this finding. Diverse neighbourhoods might be poorer than more homogeneous ones. They may have higher rates of crime, delinquency or anti-social behaviour. Through an in-depth analysis, Putnam is able to show that such is not the case. Trust and social capital are diminished in diverse areas regardless of their overall level of affluence or incidence of crime, delinquency, and other influences that could affect the results. Putnam's firm conclusion is that it is diversity as such that reduces connections with, and confidence in, others.

This conclusion is itself distinctly uncomfortable for liberals. Perhaps multiculturalism just will not work? Putnam rejects such a pessimistic conclusion. The negative effects of diversity can be overcome by a mixture of positive social change and enlightened public policy. He provides a number of encouraging examples. For instance, a generation ago the US army was divided along racial lines, but today it has become a 'colour-blind institution'. American soldiers today on average have many more inter-racial friendships than Americans as a whole.

The aim of social policy up to now has nearly always been to reduce the segregation between ethnic groups, concentrating mainly upon minorities. Putnam's research, however, strongly implies that getting all groups to identify with the community is most important. Building up community identity means trying to foster an overall sense of pride and involvement with an institution or neighbourhood. Pride in the military and identification with its goals almost certainly was a prime factor underlying the observed changes in the army.

How far do Putnam's findings, if they are valid, apply elsewhere - for example, in Europe? We do not know for certain, since the detailed materials Putnam was able to work from for the US don't exist even for individual European societies, let alone on a more general level. There are some differences between Goodhart's arguments and those of Putnam. Goodhart was talking about the welfare state on a national level, and was more tentative in his conclusions; Putnam focuses more firmly upon local communities. Yet Putnam's work does provide some backing for Goodhart's view.

I have to say that at the moment I am not wholly convinced by Putnam's arguments. He says diversity weakens social capital within a community, but what actually is a community in today's society? In an era of electronic networking, it cannot necessarily be identified with a physical neighbourhood. Moreover, he does not really explain how it is that diversity undermines the social capital of all groups living in a certain area. Goodhart's thesis, at the moment at least, is a tentative one, as he himself stresses. Sweden, for instance, is a country that has experienced a good deal of recent immigration - about 14 per cent of its population is foreign-born; but it has sustained its generous and effective welfare system, albeit with many stresses and strains.

What I am persuaded of is that political correctness should not be allowed to stand in the way of further research and analysis. If diversity does compromise solidarity, it is a fact that should be brought out in the open, not dismissed for ideological reasons. It might make multicultural ideals more difficult to realise but, as Putnam shows, it is not a reason to abandon them.